The invention relates to a high-pressure gas discharge lamps having a current-supply conductor enclosed by an insulator and extending proximate the inside wall of the outer envelope.
High pressure discharge lamps typically have an outer envelope sealed in a vacuum-tight manner and having a seal at a first end thereof, and a lamp cap provided with contacts, in which the first end of the outer envelope is fixed. A discharge vessel sealed in a vacuum-tight manner is arranged within the outer envelope and provided with an ionizable filling and with electrodes connected to a first and a second current lead-through conductor, respectively, which emanate from the discharge vessel near and remote from the first end of the outer envelope, respectively. A first and second current-supply conductors, are connected to respective contacts of the lamp cap and extend through the seal at the first end of the outer envelope to the first and the second current lead-through conductor, respectively. A part of the second current supply conductor is enveloped within the outer envelope by an insulator. Such a lamp is known, for example, from U.S. Pat. No. 4,002,940.
In high-pressure gas discharge lamps to be used in an optical system, endeavours are made to give the outer envelope the smallest possible transverse dimensions. As a result, minimal limitations are imposed on the design of the optical system. Small transverse dimensions of the outer envelope result in that the second current supply conductor extends within the outer envelope at a small distance from the outer envelope.
In certain high-pressure gas discharge lamps, it is necessary that a part of the second current supply conductor is surrounded by an insulator. This is the case if it must be prevented that small metal ions, such as ions of sodium, disappear from the filling due to the fact that U.V. radiation releases electrons from the second current supply conductor, which electrons are deposited on the discharge vessel. The negative charge on the discharge vessel is conducive to migration of sodium ions through the wall of the discharge vessel.
An insulator around the second current supply conductor may alternatively be necessary to enlarge the smallest distance between bare parts of the first and the second current supply conductor in order to reduce or exclude the risk of flash-over. This is the case in lamps in which a very high voltage is used, for example, of several kV to several tens of kV for re-igniting the lamp in the hot state after a lamp current interruption.
In a lamp in which the second current supply conductor extends within the outer envelope at a small distance therefrom, the insulator around the current supply conductor can be located at such a small distance from the outer envelope that the lamp is highly sensitive to shocks and that even during manipulation, for example packing, of the lamp the insulator is liable to break.